At present, the 3rd Generation Partnership Project (3GPP) has started up a study project of Long Term Evolution (LTE) of 3G radio interface technology. With the progression of the study, it is determined that the LTE system supports the following 2 types of radio frame structures.
A) The first type of radio frame (Type1 for short below), which is applicable to a Frequency Division Duplex (FDD) system and a TDD system, and the structure of which is as shown in FIG. 1.
As shown in FIG. 1, the frame length of Type1 is 10 ms, and the radio frame consists of 20 time slots, where the length of each time slot is 0.5 ms, and the time slots are labeled as #0˜#19 in FIG. 1. Each two consecutive time slots are defined as a subframe, and there are totally 10 subframes, i.e., Subframe i consists of Time slots #2i and #2i+1, where i=0, 1, 2, . . . , 9.
When Type1 is applied to an FDD system, because the uplink and downlink of the FDD system are separated on the frequency domain, there are 10 subframes available for both the uplink and the downlink in each period of 10 ms.
When Type1 is applied to a TDD system, there are totally 10 subframes available for the uplink and the downlink in each period of 10 ms, and each subframe is used for uplink transmission or for downlink transmission, where Subframe 0 and Subframe 5 are always allocated for downlink transmission.
The LTE system is based on the Orthogonal Frequency Division Multiplexing (OFDM) technology, the subcarrier interval is set as 15 KHz, and the corresponding OFDM symbol length is 66.67 μs; for Type1, the length of each time slot is 0.5 ms, and when a small coverage range is supported, a short cycle prefix (CP) with a length of 4.7 μs is used, and each time slot contains 7 OFDM symbols; and when a large coverage range is supported, a long CP with a length of 16.67 μs is used, and each time slot contains 6 OFDM symbols. It may be known by calculation that the CP overheads of Type1 in short CP configuration and long CP configuration are about 6.7% and 20%, respectively.
B) The second type of radio frame (Type2 for short below), which is applicable to a TDD system, and the structure of which is as shown in FIG. 2.
As shown in FIG. 2, the frame length of Type2 is 10 ms, and each frame is divided into 2 half-frames each of 5 ms. Each half-frame consists of 7 service time slots (labeled as #0˜#6 in FIG. 2) and 3 special time slots. As shown in FIG. 2, the 3 special time slots are Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS), respectively.
Each service time slot is defined as a subframe, where Subframe #0 and DwPTS are always used for downlink transmission, while Subframe #1 and UpPTS are always used for uplink transmission.
In the frame structure of Type2, the length of each service time slot is 675 μs, and when short CP configuration is employed, each service time slot consists of 9 OFDM symbols with short CP configuration, where the length of the short CP is 8.33 μs, and the length of the OFDM symbol is 66.67 μs. When long CP configuration is employed, each service time slot consists of 8 OFDM symbols with long CP configuration, where the length of the long CP is 17.71 μs, and the length of the OFDM symbol is still 66.67 μs. Specifically, it is determined by the specific requirements of the application scenario whether short CP configuration or long CP configuration is employed, for example, when a small coverage range is supported, short CP configuration is employed; when a large coverage range is supported or when a multi-cell broadcast service is carried out, long CP configuration is employed.
It can be seen from the frame structures of Type1 and Type2 that, because the frame structures are different, the CP overhead in Type2, especially in the case of short CP configuration, is relatively large, which reaches 8.33/66.67=12.49%. In an OFDM system, the CP length determines the anti-multipath capacity of the OFDM system, so a long CP is favorable to overcome multipath interference, but the system overhead may be large, and the relatively large overhead may affect the peak rate and the transmission efficiency of the system, thereby lowering the data transmission capacity. For the channel environment in a practical application, a CP of about 5 μs can meet the requirements to overcome the affect caused by multipath delay spread. Therefore, the existing CP length in Type2 may decrease the transmission efficiency.
For a TDD system, in order to avoid the interference between the uplink and downlink time slots, a GP needs to be set at the switch point from the downlink time slot to the uplink time slot, and the length of the GP equals to the time for an electromagnetic wave to propagate a distance that is twice of the cell radius, i.e., TGP=2*Rcell/C, where Rcell represents the cell radius, C represents the propagation velocity of the electromagnetic wave in vacuum (3×108 m/s).
In the prior art, the lengths of the three special time slots, DwPTS, GP and UpPTS, in the special time slot area in Type2 are fixed, as 83.33 μs, 50 μs and 141.67 μs respectively, where it may be known from the above calculation formula of TGP that the coverage radius supported by the system under such a frame structure configuration is determined by the length of the GP.
Currently, methods for adjusting the GP for different coverage ranges includes:
A) For a cell radius less than 7.5 km, the existing frame structure is used, as shown in FIG. 3, the GP length is 50 μs, and random access is carried out in the UpPTS.
B) For a medium coverage range with a cell radius larger than 7.5 km and less than 30 km, the frame structure as shown in FIG. 4 is used, and the inherent GP length of 50 μs in Type2 is insufficient, thus the GP and the UpPTS are combined to a new GP, the length of the new GP is 191.66 μs, and it may be calculated according to the relationship between TGP and Rcell that this frame structure can support a coverage range of about 29 km. At this point, random access may be carried out in Subframe #1 or any uplink time slot thereafter;
C) For a large coverage range with a cell radius larger than 30 km, the frame structure as shown in FIG. 5 is used, and at this point, the whole Subframe #1 and GP, UpPTS are combined to a new GP, the length of the new GP is 866.66 μs, and a cell coverage range over 100 km can be supported. At this point, random access is carried out in Subframe #2 or any uplink time slot thereafter.
It can be seen from the above description that, in order to support a larger coverage range of a cell, a method for lengthening a GP to contain one or more uplink time slots is employed in the prior art. However, the above configuration method is not flexible enough for supporting the coverage range; for example, for a cell with a coverage range of 40 km, Solution C is needed for its frame structure, while at most a coverage range of 120 km can be supported by Solution C, thus a great part of the GP may be wasted in the practical application, thereby decreasing the transmission efficiency of the system.
Therefore, various requirements of the coverage range cannot be flexibly supported in the prior art.